In recent years, liquid crystal display devices are often used not only in information and communications equipment but also in general electrical equipment. A liquid crystal display device is composed of a pair of substrates made of glass or the like on the surfaces of which electrodes and the like are formed, and a liquid crystal layer formed between the pair of substrates. Applying a voltage to the electrodes on the substrate causes the liquid crystal molecules to array themselves so as to vary light transmissivity, thereby displaying various images.
Such a liquid crystal display device includes an array substrate and a color filter substrate, and liquid crystal is sealed between the two substrates. Scan lines and signal lines are arranged in a matrix on the surface of the array substrate. Thin film transistors (TFTs), pixel electrodes, and auxiliary capacitor lines are disposed in the regions enclosed by the two types of lines. The TFTs are switching elements for driving the liquid crystal. The pixel electrodes serve to apply a voltage to the liquid crystal. The auxiliary capacitor lines form an auxiliary capacitor for maintaining signals. Color filters for, e.g., red (R), green (G), and blue (B), a common electrode, etc., are formed on the surface of the color filter substrate.
The auxiliary capacitor lines formed on the array substrate are provided to form auxiliary capacitors for maintaining the electric charge of signals supplied from the signal lines for a predetermined period of time. Each auxiliary capacitor employs an auxiliary capacitor line and a part of a drain electrode of a TFT or a part of a pixel electrode as its electrodes, and a gate insulating film that covers the gate electrode of the TFT as its dielectric, in order to serve as a capacitor. The auxiliary capacitor lines are generally formed of a light-blocking conductive material, such as aluminum, molybdenum, chromium, or the like.
In order to prevent crosstalk or flicker of the liquid crystal display device, the auxiliary capacitors need to have a large capacitance. However, since recent technological innovation has allowed liquid crystal display devices to become increasingly smaller and achieve higher definition, making the size of individual pixels smaller, it is practically difficult, considering the aperture ratio of each pixel, to widen the auxiliary capacitor lines to increase the auxiliary capacitance.
As a solution to the above problem, a technique as disclosed in JP-T-2005-506575 (FIGS. 8 and 9, paragraphs [0069] to [0085]) has been proposed. An array substrate 70 of a liquid crystal display device as disclosed in this exemplary related art will now be described with reference to FIG. 9. FIG. 9A is a plan view of the array substrate, and FIG. 9B is a cross-sectional view thereof taken along line IXB-IXB of FIG. 9A.
In the array substrate 70 of this liquid crystal display device, as illustrated in FIGS. 9A and 9B, scan lines 72, auxiliary capacitor lines 73, and rectangular auxiliary capacitor patterns 74 made of a conductive material such as aluminum, chromium, molybdenum, chromium nitride, molybdenum nitride, or an alloy thereof, are formed on a transparent insulating substrate 71. The scan line 72 is connected to a gate electrode G of a thin film transistor (TFT), and the auxiliary capacitor pattern 74 is connected to the auxiliary capacitor line 73.
On the insulating substrate 71, a gate insulating film 75 made of an insulating material, such as silicon nitride or silicon oxide, and having a thickness of 2500 to 4500 Å is formed so as to cover the scan line 72, the auxiliary capacitor line 73, and the auxiliary capacitor pattern 74. On the gate insulating film 75, a semiconductor pattern 76 made of amorphous silicon or the like is formed so as to overlie the gate electrode G. On a part of the semiconductor pattern 76 and the gate insulating film 75, signal lines 77 and an auxiliary-capacitor-use conductive pattern 78 (i.e., a conductive pattern for the auxiliary capacitor) made of a conductive material are formed. The signal line 77 extends in a vertical direction and serves also as a source electrode S of the TFT.
The auxiliary-capacitor-use conductive pattern 78 is formed on the same layer as the signal line 77 so as to assume the form of an island, and forms an auxiliary capacitor in conjunction with the auxiliary capacitor pattern 74 positioned below and the gate insulating film 75 in between. The auxiliary-capacitor-use conductive pattern 78 is electrically connected to a pixel electrode 79, which will be described below.
In addition, a protective insulating film 80 made of an insulating material, such as silicon nitride or silicon oxide, and having a thickness of 500 to 2000 Å covers the signal lines 77, the auxilliary-capacitor use conductive pattern 78, and the semiconductor pattern 76. The protective insulating film 80 is provided with a contact hole 81 at a position above the drain electrode D and an opening 82 at a position above the auxiliary-capacitor-use conductive pattern 78. The pixel electrode 79 is formed on the protective insulating film 80 so that the pixel electrode 79 and the drain electrode D are electrically connected to each other via the contact hole 81, and the auxiliary-capacitor-use conductive pattern 78 and the pixel electrode 79 are connected to each other via the opening 82. Thus, the auxiliary-capacitor-use conductive pattern 78 and the drain electrode D are electrically connected to each other via the pixel electrode 79. The pixel electrode 79 is formed of a transparent conductive material, such as indium tin oxide (ITO) or indium zinc oxide (IZO).
In the above technique, the pixel electrode 79 overlies the auxiliary capacitor line 73 and the auxiliary-capacitor-use conductive pattern 78, and forms an auxiliary capacitor in conjunction with the auxiliary capacitor line 73, with the protective insulating film 80 and the gate insulating film 75 interposed between the two. In addition, the pixel electrode 79 is electrically connected to the auxiliary-capacitor-use conductive pattern 78, and the auxiliary-capacitor-use conductive pattern 78 forms another auxiliary capacitor in conjunction with the auxiliary capacitor pattern 74, with the gate insulating film 75 interposed between the two. Because the thickness of the gate insulating film 75 positioned between the auxiliary-capacitor-use conductive pattern 78 and the auxiliary capacitor pattern 74 is small, a larger capacitance is secured in this case than in the case where the auxiliary capacitor is formed by the auxiliary capacitor pattern 74 and the overlying pixel electrode 79, although the overlie area is the same. Therefore, in the liquid crystal display device as disclosed in JP-T-2005-506575 (FIGS. 8 and 9 and paragraphs [0069] to [0085]), it is possible to increase the capacitance without increasing the areas of the auxiliary capacitor patterns 74 and the auxiliary capacitor lines 73, and therefore the ratio of the capacitance to the aperture ratio can be improved.
However, in the array substrate 70 of the liquid crystal display device as disclosed in JP-T-2005-506575 (FIGS. 8 and 9 and paragraphs [0069] to [0085]), each capacitor (i.e., auxiliary capacitor) employs an auxiliary-capacitor-use conductive pattern 78 and auxiliary capacitor pattern 74 as its electrodes and the gate insulating film 75 positioned between the two as its dielectric. The thickness of the gate insulating film 75, although purportedly small, is nevertheless 2500 to 4500 Å, which is not sufficiently small to secure an auxiliary capacitance sufficient for preventing display troubles, such as crosstalk, flickering, etc., without increasing the area of the auxiliary capacitor pattern 74, which is made of a light-blocking conductive material. In other words, in the array substrate 70 of the liquid crystal display device as disclosed in JP-T-2005-506575 (FIGS. 8 and 9 and paragraphs [0069] to [0085]), although an increase in the auxiliary capacitance could be accomplished by making the thickness of the gate insulating film 75 smaller, that would cause difficulty in maintaining electrical isolation between the gate electrode G and the scan line 72, which are covered by the gate insulating film 75, and other components.
As another example of related art for achieving an auxiliary capacitor with large capacitance, an array substrate of a liquid crystal display device 90 as disclosed in Japanese Patent No. 2,584,290 (Claims, page 2, column 4, line 30 to page 3, column 5, line 17, and FIGS. 1 and 2) will now be described with reference to FIGS. 10 and 11. FIG. 10 is a plan view of the array substrate as disclosed in Japanese Patent No. 2,584,290 (Claims, page 2, column 4, line 30 to page 3, column 5, line 17, and FIGS. 1 and 2), the view illustrating a portion thereof corresponding to a few pixels. FIGS. 11A to 11G are partial cross-sectional views of the array substrate of FIG. 10, the views illustrating processes for manufacturing it in sequential order. First, a pattern of an auxiliary capacitor line 92 made of indium tin oxide (ITO) is formed on an insulating substrate 91 made of a glass plate. Next, a gate metal film 93 is formed and patterned (FIG. 11A).
Then, by plasma CVD or the like, a gate insulating film 94 made of SiNx or SiOx, an amorphous semiconductor film 95 to serve as an active layer made of, e.g., a-Si, and a semiconductor film 96 (for Ohmic contact) constituted by, e.g., an n+a-Si layer doped with impurities are formed sequentially (FIG. 11B). At this time, the thickness X of the gate insulating film 94 is set to be sufficiently large, e.g., X=4000 Å, to prevent a short-circuit from occurring between the drain and gate or between the source and gate.
Next, the semiconductor film 96 (for Ohmic contact) and the amorphous semiconductor film 95 are subjected to etching using the same resist to form patterns (FIG. 11C). Then, a resist (not shown in FIG. 11) is applied that is provided with an opening pattern at a portion thereof corresponding to the position (indicated by a broken line in FIG. 10) at which the auxiliary capacitor line 92 will be overlain by a pixel electrode 97, which will be formed in a later process. Then, using etchant for the gate insulating film 94, etching is performed to cause the gate insulating film 94 to have at that position a desired smaller thickness Y=2000 Å so as to serve as an insulating film for the auxiliary capacitor (FIG. 11D).
Next, the pixel electrode 97 made of ITO is formed and patterned (FIG. 11E). Further, a metal film 98 for the drain and the source is formed and patterned (FIG. 11F), and a portion of the semiconductor film 96 (for Ohmic contact) that is present at a channel portion of the TFT is removed by etching, whereupon manufacture of the array substrate for the liquid crystal display device is completed (FIG. 11G). The liquid crystal display device 90 is obtained by arranging the array substrate manufactured by the above processes and a common electrode substrate so as to face each other with a liquid crystal material between them.
In the above known technique, the auxiliary capacitor line 92 and the pixel electrode 97 correspond to the electrodes of a capacitor, and the gate insulating film 94 positioned between the auxiliary capacitor line 92 and the pixel electrode 97 corresponds to the dielectric of the capacitor. The portion of the gate insulating film 94 that is positioned above the gate electrode 93 has a thickness X=4000 Å, whereas the insulating film above the auxiliary capacitor line 92 has a thickness Y=2000 Å. Therefore, short-circuits are less likely to occur between the drain and the gate or between the source and the gate, and at the same time, a sufficient auxiliary capacitance can be secured without the need to increase the area of the auxiliary capacitor line 92.
In the above-described array substrate of the liquid crystal display device 70 as disclosed in JP-T2005-506575 (FIGS. 8 and 9 and paragraphs [0069] to [0085]), securing an auxiliary capacitance sufficient to prevent display troubles requires the auxiliary capacitor pattern to have a large area, which results in the aperture ratio being reduced. In addition, because the TFT and the auxiliary-capacitor-use conductive pattern 78 are present as light-blocking members within the pixel, the aperture ratio is further reduced. On the other hand, in the above-described array substrate of the liquid crystal display device 90 as disclosed in Japanese Patent No. 2,584,290 (Claims, page 2, column 4, line 30 to page 3, column 5, line 17, and FIGS. 1 and 2), the portion of the gate insulating film that is positioned above the auxiliary capacitor line is partially removed by etching to obtain a thinner insulating film, and thereby the auxiliary capacitance is increased while maintaining electrical isolation between the gate electrode and the scan line, which are covered by the gate insulating film, and other components. However, it is difficult to control the amount of etching so as to obtain the desired thickness by partial removal of the auxiliary capacitor line portion of the gate insulating film, and it is difficult to maintain uniformity of the thickness of the auxiliary capacitor line portion of the gate insulating film across liquid crystal display devices.
In addition, in the array substrate of the liquid crystal display device 90 as disclosed in Japanese Patent No. 2,584,290 (Claims, page 2, column 4, line 30 to page 3, column 5, line 17, and FIGS. 1 and 2), after the auxiliary capacitor line 92 made of ITO is formed and patterned on the insulating substrate 91 formed of a glass plate, the gate metal film 93 is formed and patterned to form the scan line and the gate electrode. Thus, the number of processes is increased, resulting in reduction in production efficiency. Further, out of consideration for mask displacement or the like, there must be a long distance between the pixel electrode 97 and the metal film 98 for the source, so that it is impossible to arrange the pixel electrode so as to overlie the TFT. As a result, the aperture ratio is reduced. Therefore, it is difficult to use this array substrate as a method for forming auxiliary capacitors in the liquid crystal display devices of recent years, which have a relatively small pixel area and high definition.